146 NERVOUS SYSTEM OF VERTEBRATES. 



retina and branches in relation with the dendrites of cells form- 

 ing the inner nuclear layer (Figs. 71, 72). These cells in turn 

 set up connections with the elements of the ganglionic cell layer. 

 The fibers from the ganglionic cells form a fiber layer on the 

 outer, concave surface of the retina. The fibers collect toward a 

 point near the center of the retina, dip into its substance and 

 pierce it to its inner surface. From this surface of the retina 

 the fibers pass along the optic stalk to the brain. This arrange- 

 ment is brought about during development by a change of 

 form of the retina and shifting of position of the stalk. At first 

 the stalk is at the ventral border of the retina and the fibers 

 run to the ventral border and thence to the brain in the stalk. 

 Later the retina grows in such a way as to bring the attachment 

 of the optic tract nearer its center. Finally, in all vertebrates 

 except cyclostomes and some fishes and urodule amphibia, the 

 hollow stalk degenerates and leaves the optic tract as the only 

 connection of the retina with the brain. It should be noticed 

 now that this optic tract, although commonly called the optic 

 nerve, differs from all peripheral nerves in several ways, (i) 

 The cells from which its fibers arise are derived from the wall of 

 the brain. (2) They form part of a many-layered nervous 

 structure whose development and histology suggest comparison 

 with the brain wall. (3) Although the fibers carry afferent 

 impulses they enter the ventral wall of the brain. Indeed, it has 

 long been recognized that this is not a peripheral nerve but a 

 central nerve tract and that the retina is a part of the brain wall. 

 When the optic tracts arrive at the ventral wall of the dien- 

 cephalon (or before in bony fishes) they decussate, forming the 

 optic chiasma. As a general rule the fibers from one retina pass 

 to the opposite side of the brain, but in mammals a complete 

 decussation of the optic tracts is rare. In different mammals a 

 variable part of the fibers remain uncrossed and in man approxi- 

 mately one-third enter the same side of the brain as the eye from 

 which they come. In vertebrates below the mammals most authors 

 agree that the decussation is total. The beginnings of partial 

 decussation are seen, however, in fishes. Golgi preparations of 

 the chiasma of the sturgeon show a considerable number of fibers 



